1. Field of the Invention
The present invention relates to a light-emitting device (LED), and more particularly to a LED that employs a group III nitride compound semiconductor which provides a wide range of color reproduction and emits white light.
2. Background Art
A variety of white-light-emitting LEDs (light-emitting devices using group III nitride compound semiconductor) are generally known in the art. For example, Isamu AKASAKI describes such an LED in his book entitled Power of Blue-Light-Emitting Device, K books series 122, Kogyo Chosakai Publishing Co., Ltd., 1997, (hereinafter to as Reference 1) Japanese Patent Application Laid-Open (kokai) No. 5-152609 (Title: xe2x80x9cLight-Emitting Diodexe2x80x9d) (hereinafter referred to as Reference 2) discloses another such LED.
FIG. 4 shows a cross-sectional view of a conventional semiconductor white-light-emitting device 400, which is disclosed in the aforementioned Reference 1. The conventional semiconductor light-emitting device 400 comprises a group III element nitride compound semiconductor blue-light-emitting device (diode chip), placed in a metal-made cup. A YAG phosphor converts the blue light, introduced around the diode chip, into a yellow light.
FIG. 5 is a graph showing an emission spectrum of the conventional semiconductor light-emitting device 400. The light emitted directly from the chip has a sharp peak at approximately 450 nm, and the light obtained from the phosphor molecules has a broad peak at approximately 550 nm.
The chromaticity of the light emitted from the white-light-emitting LED (conventional semiconductor light-emitting device 400) can be modified by adjusting the amount or composition of the phosphor employed.
FIG. 6 is a chromaticity diagram showing a color reproduction area of the conventional semiconductor light-emitting device 400. As shown in the diagram, an LED emitting light of arbitrary chromaticity can be obtained by adjusting the amount or composition of the phosphor employed. In FIG. 6, the arbitrary chromaticity is included within the central sector of the chromaticity diagram.
In addition, the aforementioned Reference 1 discloses a white-light-emitting xe2x80x9c3-in-1 full color LED.xe2x80x9d In this LED, a semiconductor light-emitting device containing gallium aluminum arsenic (GaAlAs) serves as a red-light-emitting LED chip.
As illustrated in FIG. 6, the conventional semiconductor light-emitting device""s range of color reproduction area is insufficient. Therefore, when the light-emitting device is used in a lamp, the task of highlighting green or red images becomes difficult. Thus, a conventional 1-chip semiconductor light-emitting device cannot be used effectively to produce a full color reproduction when installed within a lamp having a wide range of color reproduction area.
In addition, to produce the aforementioned white-light-emitting xe2x80x9c3-in-1 full color LEDxe2x80x9d, several chips are required in a conventional device. This induces cumbersome and time-consuming production steps, thereby increasing unfavorably the production cost of the conventional devices.
Moreover, in order to produce a red-light-emitting chip employed in conventional semiconductor light-emitting devices such as the aforementioned xe2x80x9c3-in-1 full color LED,xe2x80x9d arsenic (As) is added to a red-light-emitting semiconductor layer. However, when products containing an arsenic compound are manufactured on a large scale, environmental and ecological concerns must be addressed. These concerns generate problems in productivity and increases the cost to build regulatory-compliant production facilities.
The present invention overcomes the aforementioned and other problems. Thus, an object of the present invention is to provide a white-light-emitting LED exhibiting a sufficiently wide color reproduction area and is capable of full color reproduction. In addition, the present LED does not contain an arsenic compound and is produced by a comparatively low-cost facility that requires no special environmental considerations.
Accordingly, the present invention provides a light-emitting device that employs a group III nitride compound semiconductor containing stacked semiconductor layers which have a quantum well structure. The group III element nitride compound is represented by the formula, (AlxGa1xe2x88x92x)yIn1xe2x88x92yN (0xe2x89xa6xxe2x89xa61; 0xe2x89xa6yxe2x89xa61). At least three of the well layers have compositional proportions which differ from one another. An acceptor impurity and a donor impurity are added to at least one well layer. Furthermore, The chromaticity of light, emitted from each of at least three well layers, is controlled so that the well layers emit lights of different chromaticities so that a white light can be produced. The lights having different chromacities are emitted from each layer and are combined together. The composition of this mixture of light is adjusted and radiated from a light-extraction surface so as to obtain a white light.
Each of the aforementioned well layers may be a single quantum well layer or a multiple quantum well layer and may further contain a thin barrier layer therein. These well layers may or may not interfere with one another.
Preferably, the light-emitting device using a group III nitride compound semiconductor has a red-light-emitting well layer in which the compositional proportion of AlyIn1xe2x88x92yN is adjusted to satisfy 0xe2x89xa6yxe2x89xa60.1.
Preferably, the light-emitting device using a group III nitride compound semiconductor includes a blue-light-emitting well layer and a green-light-emitting well layer which are formed of In1xe2x88x92yGayN (0.7xe2x89xa6y less than 1) to which no impurity is added.
The blue-light-emitting well layer may be a well layer emitting bluish purple light having a wavelength of 380-455 nm, which is slightly shorter than the wavelength of the blue light of 455-485 nm.
Preferably, the acceptor concentration and the donor concentration are adjusted in the range from 1xc3x971017/cm3 to 1 xc3x971021/cm3, respectively.
Preferably, the acceptor impurity may be, for example, zinc (Zn), beryllium (Be), calcium (Ca), strontium (Sr), barium (Ba), or magnesium (Mg).
Preferably, the donor impurity may be, for example, carbon (C), silicon (Si), tin (Sn), sulfur (S), selenium (Se), or tellurium (Te).
Preferably, the well layers are stacked in descending order such that the emission wavelengths of the layers decrease toward the light-extracting surface.
Preferably, the weighted mean coordinates of the chromaticity coordinates in a chromaticity diagram weighted by the emission intensity of light emitted from the well layers are adjusted to approximately (⅓, ⅓) by controlling the thickness of each well layer, compositional proportions of each well layer, species and concentration impurities added to each well layer, or the number of layers emitting light of the same wavelength.